WO2005088345 discloses an X-ray detector with a photo-sensitive detector layer above which a scintillation layer for the conversion of X-rays into photons is disposed.
Semiconductor X-ray detectors are built by combining a scintillator or a photoconductor with a semiconductor imaging device. In the first case, the scintillator converts X-ray quanta to light and the light is converted in the semiconductor imaging device into electrical charges, which are finally read out to yield an image. In the second case, the photoconductor immediately converts absorbed X-ray quanta into electric charges, which are collected by the imager and which are finally read out to yield the image.
The imaging devices can include flat panel imagers based on amorphous silicon or polycrystalline silicon with photodiodes or charge collection electrodes in the pixel matrix. Other forms of imaging devices are charge coupled devices (CCD) and imagers based on complementary metal oxide semiconductors (CMOS imagers). The latter imaging devices often take the form of so called active pixel sensors because they contain transistors in the pixel to amplify the signals.
A common problem with semiconductor X-ray detectors is that not all X-ray quanta are absorbed in the conversion layer, i.e. in the scintillator or photoconductor.
The X-ray quanta which go through the conversion layer can, with a low probability, cause an interaction in the semiconductor imaging device, depositing a usually large amount of charge in a single pixel. This results in a number of very bright pixels in each image. In the following text, such an event will be referred to as “parasitic direct detection event”.
The intended process of conversion of X-ray quanta in the conversion layer and collection of light or charges by the imager usually happens much more frequent but with a much lower charge yield.
Hence, in addition to the normal image, a small number of very bright pixels can be observed in each image. The number and position of bright pixels change from image to image and substantially disturb the intended image.